Longest--Christmas, 2011 we cruised on a reservation we'd made in 2005. I just kept pushing it forward.
There is also a recent thread about the shortest, basically a few people have booked the next cruise for various reasons. Personally we book about a year in advance and usually when the first bookings come out. We have had several cruises in a month's time but they have all been booked in advance. But with the IGT,OGT,VGT rates last minute bookings/rebookings seem much more possible.